The present invention relates to a liquid crystal display device, and in particular to a technology useful for application to a backlight disposed behind the liquid crystal display device (hereinafter referred to as a direct-light type backlight). TFT (Thin Film Transistor) liquid crystal display modules are widely used as display devices for notebook personal computers and others. Those liquid crystal display modules comprise a liquid crystal display panel having a drain driver and a gate driver disposed therearound and a backlight for illuminating the liquid crystal display panel. Backlights are divided broadly into side-light type backlights and direct-light type backlights. Generally, the liquid crystal display modules used as display devices for notebook personal computers employ the side-light type backlights. In recent years, liquid crystal display modules have been made large-sized with their screen size increased, and they have been used as display devices for monitors. Such large-sized and large-screen liquid crystal display modules for use in monitors employ direct-light type backlights capable of providing high brightness.
FIG. 11 is an exploded perspective view illustrating a rough configuration of a liquid crystal display module. As shown in FIG. 11, the liquid crystal display module is comprised of an upper frame 1 made of a metal peripheral frame (also called a shield case, an upper case or a metal upper frame), a liquid crystal panel 2 and a backlight BL. The liquid crystal panel is provided with a pair of substrates (made of light-transmissive electrically-insulating material, for example, glass) sandwiching a liquid crystal layer therebetween, a drain circuit board DPCB disposed around the pair of substrates, and two gate circuit boards GPCB. Mounted on each of the circuit boards are tape carrier packages DTCP or GTCP having plural liquid-crystal-driving semiconductor integrated-circuit elements (driving ICs) mounted thereon by using tape automated bonding (TAB). Coupled to the driving ICs are flexible circuit boards DFPC for supplying signals or electric power to the driving ICs and connection flexible circuit boards GFPC are provided for connecting the drain circuit board DPCB with the gate circuit boards GPCB.
The pair of substrates are superposed one upon another with a specified spacing therebetween, and then they are sealed together by using a sealing agent in the form of a picture-frame and sandwiched between the vicinities of peripheries of the two opposing substrates. Next liquid crystal material is filled into a space inside the two sealed substrates via a liquid crystal filling hole made in the sealing agent, and then the liquid crystal filling hole is sealed off. Then the liquid crystal panel 2 is completed by attaching polarizers on the outside surfaces of the two substrates.
The flexible circuit boards DFPC are electrically connected to a circuit board Tcon having integrated circuit elements such as a timing converter and others disposed under the backlight BL. Reference numeral 5 in FIG. 11 denotes a cover for the circuit board Tcon.
FIG. 12 is an exploded perspective view illustrating a rough configuration of the direct-light type backlight BL shown in FIG. 11. As shown in FIG. 12, the direct-light type backlight BL shown in FIG. 11 is comprised of a reflective plate 10, plural cold cathode fluorescent lamps CCFL, a light diffusing plate (relatively thick, 2 mm, for example) 11, a lower light diffusing sheet (relatively thin, several hundreds μm, for example) 12, a prismatic sheet (also called a brightness improving sheet) 13, an upper light diffusing sheet (relatively thin, several hundreds μm, for example) 14 arranged in the order shown in FIG. 12, between a mold 7 made of synthetic resin and a lower frame 3 made of metal. Incidentally, in some cases two prismatic sheets are employed. The liquid crystal display module is completed by sandwiching and fixing the assembled liquid crystal panel 2 between the upper frame 1 and the backlight BL.
As explained above, the direct-light type backlight BL is provided with the plural cold cathode fluorescent lamps CCFL, the plural optical members (the light diffusing plate, the light diffusing sheets, the brightness improving sheet, and others) disposed over the plural cold cathode fluorescent lamps CCFL, and the reflective plate having a reflective surface which reflects toward the liquid crystal display panel the light radiated from the lamps CCFL toward the side of the lamps CCFL opposite from the liquid crystal display panel. However, there is a problem in that the cost of the optical members including the light diffusing plate, the light diffusing sheets, the brightness improving sheets and others account for a large percentage in a range of 40% to 70% of the cost of the direct-light type backlight. To solve this problem, the direct-light type backlights which do not employ the brightness improving sheet are disclosed by Patent Application Laid-Open Publication Nos. 2004-6256, Hei 6-273763, and 2004-31312.